1. Field of the Invention
The present invention relates to a mirror surface accuracy measuring device which is applied to a reflector antenna such as a large diameter radio telescope for astronomic observation using a millimeter radio wave or a submillimeter radio wave and in which mirror surface accuracy of the reflector antenna is measured. Also, the present invention relates to a mirror surface control system of the reflector antenna in which the adjustment of a mirror surface of a main reflector composed of a plurality of mirror panels is improved according to the mirror surface accuracy measured by the mirror surface accuracy measuring device.
2. Description of Related Art
A reflector antenna such as a radio telescope is used to perform astronomic observation by reflecting a radio wave radiated from a faraway celestial body on a reflector, converging the reflected radio wave and receiving the converged radio wave in a primary radiator. A radio wave radiated from a celestial body is propagated while spreading like a spherical wave. However, because an observation point is far away from the celestial body, the radio wave of the celestial body is incident like a plane wave on the reflector antenna. In case of the astronomic observation using the radio telescope, to efficiently converge the radio wave incident like a plane wave on the primary radiator, a uniform aperture phase distribution is required. This aperture phase distribution directly depends on the mirror surface accuracy of the main reflector. Therefore, it is very important to heighten the mirror surface accuracy of the reflector antenna for the purpose of improving the observation performance of the reflector antenna.
To measure the mirror surface accuracy of the reflector antenna, a mechanical measuring technique using a private gauge or a range-angle measuring unit and an electrical measuring technique such as a radio holography method have been used as a prior art. In cases where the mechanical measuring technique is used to measure the mirror surface accuracy of the reflector antenna, because a measurement error in the use of a measurement jig depends on the manufacturing accuracy and positioning accuracy of the measurement jig, it is difficult to significantly measure the mirror surface accuracy required of the reflector antenna such as a large diameter radio telescope which is used to perform astronomic observation with a millimeter radio wave or a submillimeter radio wave. Therefore, in a general case, the mechanical measuring technique is used for the initial adjustment of the mirror surface of the large diameter radio telescope used for the astronomic observation with a millimeter radio wave or a submillimeter radio wave, and the radio holography method of the electrical measuring technique is used for the final adjustment of the mirror surface.
FIG. 7 is a constitutional view showing the configuration of a conventional mirror surface control system in which mirror surface accuracy of a reflector antenna is measured and controlled according to a radio holography method. This conventional mirror surface control system is disclosed in xe2x80x9cMeasurement of Mirror Surface Accuracy of 45m Radio Wave Telescope based on Radio Holography Methodxe2x80x9d, written by M. Ishiguro, K. Morita, S. Hayashi, T. Masuda, E. Ebisu and S. Betsudan, Technical Report Vol. 62, No. 5, pp. 69-74 of Mitsubishi Electric Corporation, in 1988.
In FIG. 7, 1 indicates a reflector antenna. 2 indicates a geostationary satellite. 3 indicates a collimation antenna mounted on the geostationary satellite 2 and functioning as a transmitted wave source. 4 indicates a transmitted radio wave radiated from the collimation antenna 3. 5 indicates a main reflector of which the mirror surface accuracy is measured. 5a indicates each of a plurality of mirror panels composing the main reflector 5. 5b indicates each of a plurality of actuators for changing setting positions and attitudes of the mirror panels 5a. 5c indicates a backing structure on which the mirror panels 5a and the actuators 5b are supported. 6 indicates a primary radiator in which a radio wave reflected and converged on the main reflector 5 is received. 7 indicates a receiver to which the radio wave is fed from the primary radiator 6. 8 indicates each of a plurality of support struts. 9 indicates radiation field distribution data obtained in the receiver 7. 10 indicates an antenna attitude signal. An attitude of the reflector antenna 1 is changed according to the antenna attitude signal 10 to obtain the radiation field distribution data 9 corresponding to an attitude of the reflector antenna 1. 11 indicates a radio holography processor in which the Fourier transformation is performed to calculate an aperture distribution from the radiation field distribution data 9 and the antenna attitude signal 10. 12 indicates a mirror surface accuracy processor in which the mirror surface accuracy of the main reflector 5 is calculated from the aperture distribution obtained in the radio holography processor 11. 13 indicates a mirror surface control device which controls the actuators 5b according to the mirror surface accuracy obtained in the mirror surface accuracy processor 12 to adjust setting positions and attitudes of the mirror panels 5a of the main reflector 5. 14 indicates an actuator control signal. 15 indicates a reference antenna in which a reference of the radiation field distribution data 9 is measured.
Next, an operation of the conventional mirror surface control system will be described below.
To measure the mirror surface accuracy of the main reflector 5, a radio wave is used for the reflector antenna 1. Therefore, a transmission source position of the radio wave is placed sufficiently far away from the reflector antenna 1a in the same manner as the geostationary satellite 2. Also, in place of the geostationary satellite 2, in cases where a certain on-ground position sufficiently far away from the reflector antenna 1 is set as a transmission source position of the radio wave, the on-ground position is determined on condition that the reflection of the radio wave on the earth is reduced due to geographical features. A radiation field distribution of the transmitted wave 4 on the reflector antenna 1 is obtained by receiving the transmitted wave 4 while changing the attitude of the reflector antenna 1 in two dimensions.
Therefore, the radiation field distribution data 9 and the antenna attitude signal 10 indicating the attitude of the reflector antenna 1 are measured in a pair. Because a relationship between the radiation field distribution and the aperture distribution of the transmitted wave 4 on the main reflector 5 is expressed by Fourier transformation, the radiation field distribution data 9 is sent to the radio holography processor 11, the calculation processing such as fast Fourier transformation is performed for the radiation field distribution data 9 and the antenna attitude signal 10, and the aperture distribution on the main reflector 5 is calculated. A phase term of the calculated aperture distribution expresses an aperture phase distribution and corresponds to the unevenness of the mirror surface of the main reflector 5. In the mirror surface accuracy processor 12, the aperture phase distribution is converted in used wavelength equivalent, and a distribution of degrees of deformation shifted from an ideal shape of the mirror surface is obtained. Therefore, the mirror surface accuracy of the main reflector 5 can be estimated. In addition, the setting positions and attitudes of the mirror panels 5a composing the main reflector 5 are corrected by the actuators 5b in the mirror surface control device 13, and the mirror surface accuracy of the main reflector 5 is improved.
In general, in view of antenna gain, it is required that the mirror surface accuracy of the main reflector 5 is equal to or lower than {fraction (1/20)} of a wavelength of a radio wave (for example, a radio wave radiated from a celestial body) used for the astronomic observation. In case of the reflector antenna 1 having a large diameter, because the reflector antenna 1 is used for the astronomic observation in a frequency band of a millimeter wave or a submillimeter wave having a shorter wavelength, it is required to produce the main reflector 5 with high mirror surface accuracy. Therefore, to measure the mirror surface accuracy of the main reflector 5 with higher measuring accuracy, it is required to heighten the frequency of a radio wave used for the measurement of the mirror surface accuracy.
However, because the conventional mirror surface control system of the reflector antenna 1 has the above-described configuration, frequencies of radio waves possible to be radiated from the geostationary satellite 2 as the transmitted wave 4 for the measurement of the mirror surface accuracy of the main reflector 5 are limited to a certain frequency band. Therefore, a problem has arisen that the measuring accuracy for the mirror surface accuracy of the main reflector 5 cannot be sufficiently heightened.
Also, in cases where a transmitted wave source is disposed on the ground or in cases where a radio star is used as a transmitted wave source, the frequency of a radio wave used for the measurement of the mirror surface accuracy can be arbitrarily selected. However, in cases where the mirror surface accuracy of the main reflector 5 is measured by using a measuring radio wave such as a millimeter wave or a submillimeter wave of a frequency band corresponding to a short wavelength, the measuring radio wave is considerably attenuated during the propagation of the measuring radio wave. Therefore, it is difficult to sufficiently get a dynamic range for the measurement of the mirror surface accuracy, and a measuring angle range allowed for the significant measurement of the radiation field distribution is narrowed. In general, in cases where the aperture distribution is calculated from the radiation field distribution by using the Fourier transformation, a degree of resolution of the aperture distribution is almost inversely proportional to a measuring range of the radiation field distribution for paraxial rays. Therefore, in cases where a measuring angle range allowed for the significant measurement of the radiation field distribution is narrow, a problem has arisen that the resolution of the aperture distribution is insufficient. Also, because a millimeter wave or a submillimeter wave is used for the astronomic observation using a large diameter radio telescope, a size of each mirror panel is often reduced in view of mechanical manufacturing accuracy. In this case, it is important to obtain the aperture distribution at high resolution.
Also, in case of the radio holography method, it is required to measure the amplitude and phase of the radiation field distribution. However, in cases where a millimeter wave or a submillimeter wave of a very high frequency band is used, it is difficult to measure the phase of the radiation field distribution. Also, because it is required to prepare a two-dimensional map of the aperture distribution, it is required to measure the radiation field distribution in two dimensions. In this case, it takes a comparatively long time to measure the radiation field distribution in two dimensions, and the radiation field distribution is fundamentally measured in the outdoor environment. Therefore, a problem has arisen that the mirror surface accuracy of the main reflector 5 is changed during the measurement due to the influence of temperature or wind outdoors.
In contrast, in cases where the mirror surface accuracy of the main reflector 5 is measured at a very short distance, it is not required to measure the radiation field distribution corresponding to a long distance, but it is required to directly measure the aperture distribution on the main reflector 5 by using a probe. In this case, it is required to mechanically scan a plane surface, a cylindrical surface or a spherical surface of the main reflector 5 with the probe for the measurement of the mirror surface accuracy. However, because it is required to scan an area wider than that of the main reflector 5, in case of the large diameter radio telescope using a millimeter wave or a submillimeter wave, it is substantially very difficult to accurately scan a wide area. Therefore, a problem has arisen that the measuring accuracy depends on the scanning accuracy of the probe and is lowered.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional mirror surface control system of the reflector antenna, a mirror surface accuracy measuring device and a mirror surface control system of a reflector antenna in which a radio wave of a high frequency difficult to use in the prior art is usable, an aperture distribution is obtained at high resolution even in a case of a narrow angle range in effective measurement of a radiation field distribution, mirror surface accuracy based on the measurement of only amplitude of the radiation field distribution is estimated and the mirror surface accuracy of the reflector antenna is measured with high accuracy.
The object is achieved by the provision of a mirror surface accuracy measuring device of a reflector antenna, including a collimation antenna arranged at an interval of a prescribed distance from a reflector antenna, radiation field distribution measuring means for measuring a radiation field distribution of the reflector antenna for the prescribed distance while controlling an attitude of the reflector antenna, a mirror panel radiation field distribution holding device for holding a plurality of panel radiation field distributions of a plurality of mirror panels composing a main reflector of the reflector antenna as pre-measured data, complex excitation coefficient calculating means for calculating a complex excitation coefficient of each mirror panel of the main reflector according to the radiation field distribution of the reflector antenna, the panel radiation field distribution of the mirror panel and an antennal attitude signal which indicates the attitude of the reflector antenna controlled by the radiation field distribution measuring means, and mirror surface accuracy calculating means for calculating a mirror surface error of each mirror panel and mirror surface accuracy of the main reflector according to the complex excitation coefficients of the mirror panels of the main reflector.
Therefore, the complex excitation coefficients of the mirror panels can be obtained to express the radiation field distribution of the reflector antenna in a combination of the panel radiation field distributions of the mirror panels composing the main reflector, and the mirror surface errors of the mirror panels can be obtained. As a result, in cases where a radio wave such as a millimeter wave or a submillimeter wave of a frequency band corresponding to a very short wavelength is selected as a radio wave used for the measurement of the radiation field distribution of the reflector antenna, even though the observation area usable for the significant measurement of the radiation field distribution of the reflector antenna is small, a map of the mirror surface errors having degrees of resolution corresponding to sizes of the mirror panels can be obtained, and the measurement of the mirror surface accuracy of the main reflector can be performed with high accuracy.
Also, a plurality of observation points can be arbitrarily selected to measure the radiation field distribution of the reflector antenna, and it is only required that the number of observation points for the radiation field distribution of the reflector antenna is higher than the number of mirror panels composing the main reflector. Accordingly, a measuring time of the mirror surface accuracy of the main reflector can be comparatively shortened, and influence of temperature and wind in the measurement on the measured values can be reduce. Also, even though only the amplitude of the radiation field distribution of the reflector antenna is measured, the mirror surface accuracy of the main reflector can be estimated.
The object is also achieved by the provision of a mirror surface accuracy measuring device of a reflector antenna, including a collimation antenna arranged at an interval of a prescribed distance from a reflector antenna, radiation field distribution measuring means for measuring a radiation field distribution of the reflector antenna for the prescribed distance of the collimation antenna while controlling an attitude of the reflector antenna, a virtual mirror panel radiation field distribution calculating device for dividing a main reflector of the reflector antenna into a plurality of virtual mirror panels and calculating a panel radiation field distribution of each virtual mirror panel, complex excitation coefficient calculating means for calculating a complex excitation coefficient of each virtual mirror panel of the main reflector according to the radiation field distribution of the reflector antenna, the panel radiation field distribution of the virtual mirror panel and an antennal attitude signal which indicates the attitude of the reflector antenna controlled by the radiation field distribution measuring means, and mirror surface accuracy calculating means for calculating a mirror surface error of each virtual mirror panel and mirror surface accuracy of the main reflector according to the complex excitation coefficients of the virtual mirror panels of the main reflector.
Therefore, the complex excitation coefficients of the virtual mirror panels can be obtained to express the radiation field distribution of the reflector antenna in a combination of the panel radiation field distributions of the virtual mirror panels composing the main reflector, and the mirror surface errors of the virtual mirror panels can be obtained. As a result, even though an observation area usable for the measurement of the radiation field distribution of the reflector antenna is small, a map of the mirror surface errors having degrees of resolution corresponding to sizes of the virtual mirror panels can be obtained. In particular, because sizes of the virtual mirror panels can be arbitrarily determined, a map of the mirror surface errors can be obtained at high resolution.
Also, it is not necessarily required that the main reflector of the reflector antenna is composed of a plurality of mirror panels, and it is not required that the panel radiation fields of a plurality of mirror panel actually composing the main reflector are measured. Therefore, a total time required to measure the mirror surface accuracy of the main reflector can be shortened.
The object is also achieved by the provision of a mirror surface control system of a reflector antenna, including a collimation antenna arranged at an interval of a prescribed distance from a reflector antenna, radiation field distribution measuring means for measuring a radiation field distribution of the reflector antenna for the prescribed distance of the collimation antenna while controlling an attitude of the reflector antenna, a mirror panel radiation field distribution holding device for holding a plurality of panel radiation field distributions of a plurality of mirror panels composing a main reflector of the reflector antenna as pre-measured data, complex excitation coefficient calculating means for calculating a complex excitation coefficient of each mirror panel of the main reflector according to the radiation field distribution of the reflector antenna, the panel radiation field distribution of the mirror panel and an antennal attitude signal which indicates the attitude of the reflector antenna controlled by the radiation field distribution measuring means, mirror surface accuracy calculating means for calculating a mirror surface error of each mirror panel and mirror surface accuracy of the main reflector according to the complex excitation coefficients of the mirror panels of the main reflector, and mirror surface control means for controlling and correcting a plurality of setting positions of the mirror panels of the main reflector according to the mirror surface errors of the mirror panels.
Therefore, the complex excitation coefficients of the mirror panels are obtained to express the radiation field distribution of the reflector antenna in a combination of the panel radiation field distributions of the mirror panels composing the main reflector, a map of the mirror surface errors is obtained at degrees of resolution corresponding to sizes of the mirror panels, and a plurality of setting positions of the mirror panels are adjusted according to the map of the mirror surface errors. Accordingly, the main reflector can be obtained with high mirror surface accuracy.
The object is also achieved by the provision of a mirror surface control system of a reflector antenna, including a collimation antenna arranged at an interval of a prescribed distance from a reflector antenna, radiation field distribution measuring means for measuring a radiation field distribution of the reflector antenna for the prescribed distance of the collimation antenna while controlling an attitude of the reflector antenna, a virtual mirror panel radiation field distribution calculating device for dividing a main reflector of the reflector antenna into a plurality of virtual mirror panels and calculating a panel field distribution of each virtual mirror panel, complex excitation coefficient calculating means for calculating a complex excitation coefficient of each virtual mirror panel of the main reflector according to the radiation field distribution of the reflector antenna, the panel radiation field distribution of the virtual mirror panel and an antennal attitude signal which indicates the attitude of the reflector antenna controlled by the radiation field distribution measuring means, mirror surface accuracy calculating means for calculating a mirror surface error of each virtual mirror panel and mirror surface accuracy of the main reflector according to the complex excitation coefficients of the virtual mirror panels of the main reflector, and mirror surface control means for controlling and correcting a plurality of setting positions of a plurality of mirror panels of the main reflector according to the mirror surface errors of the virtual mirror panels.
Therefore, the complex excitation coefficients of the virtual mirror panels are obtained to express the radiation field distribution of the reflector antenna in a combination of the panel radiation field distributions of the virtual mirror panels virtually composing the main reflector, a map of the mirror surface errors is obtained at degrees of resolution corresponding to sizes of the virtual mirror panels, and a plurality of setting positions of the mirror panels of the main reflector are adjusted according to the map of the mirror surface errors of the virtual mirror panels. Accordingly, the main reflector can be obtained with high mirror surface accuracy.